This invention relates to processes for the separation of desirable minerals from undesirable minerals.
In the processing of mineral-containing ores, it is necessary to remove undesirable minerals called gangue from the desired minerals. One method of accomplishing this goal is to depress the flotation of a particular mineral during the normal flotation process. In mineral flotation systems, it is common to depress or hold down the undesirable gangue materials while floating the desirable mineral or minerals. In differential or reverse flotation systems, it is common to depress or hold down the desired mineral or minerals while floating the undesirable gangue. That is, the normal flotation system where the desired mineral or minerals are floated and the gangue remains behind is reversed.
In a typical ore flotation scheme, the ore is ground to a size sufficiently smaller to liberate the desired mineral or minerals from the undesired gangue. An additional step in the flotation process involves the removal of the ultra-fine particles by desliming. Ultra-fine particles are generally defined as those less than 5 to 10 microns in diameter. The desliming process may be accompanied by or followed by a flocculation step or some other type of settling step such as the use of a cyclone separating device. This step is followed by a flotation step wherein gangue materials are separated from the desired mineral or minerals in the presence of collectors and/or frothers.
Depression is conventionally accomplished by the use of one or more depressing agents during the flotation step. The depressing agent or the depressant, when added to the flotation system, exerts a specific action on the material to be depressed thereby preventing it from floating. Various theories have been put forth to explain this phenomenon. Some of these include: that the depressants react chemically with the mineral surface to produce insoluble protective films of a wettable nature which fail to react with collectors; that the depressants, by various physical-chemical mechanisms, such as surface adsorption, mass-action effects, complex formation or the like, prevent the formation of the collector film; that the depressants act as solvents for an activating film naturally associated with the mineral; and that the depressants act as solvents for the collecting film. These theories appear closely relaed and the correct theory may eventually be found to involve elements of most or all of these and more.
It has been conventional in non-sulfide flotation systems to use naturally derived substances such as starches, dextrins and gums as depressants. However, the presence of these substances in waste water streams increases the biodegradable oxygen demand and the chemical oxygen demand and therefore creates pollution control problems. Further, in some countries, there is a prohibition against using substances such as starch which have food value in this type of commercial application. In addition, starch-type depressants require complex preparation of the reagent which results in the reagent being susceptible to bacterial decomposition and therefore monitoring of the reagent is required during storage.
Synthetic depressants have been developed that are generally useful in the separation of gangue from desirable minerals. U.S. Pat. Nos. 4,360,425 and 4,289,613 describe the use of low molecular weight polymers, copolymers and terpolymers as depressants in mineral ore flotation. U.S. Pat. No. 2,740,522 describes the use of water-soluble, anionic, linear, addition polymers of a monoethylenically unsaturated compound and the water-soluble salts thereof to depress the flotation of gangue. U.S. Pat. Nos. 3,929,629 teaches that polymers of water soluble acrylamide homopolymers or copolymers thereof with acrylic or methacrylic acid or salts thereof are useful as gangue depressants in froth flotation processes designed to treat cassiterite ore.
One of the problems associated with existing depressants is that the depressants have differing levels of effectiveness depending on the conditions under which they are used and the mineral and gangue which are to be separated. What is needed are depressants which, while generally useful in mineral processing, meet specific needs which exist within the mining industry. Further, what is needed are depressants which effectively depress the flotation of desired mineral or minerals in reverse flotation processes.
An additional problem exists concerning the use of depressants in reverse or differential flotation systems. Because no system for depressing minerals is ideal, some portion of the desired minerals will be inadvertently floated away with the gangue. That portion of the valuable mineral or minerals that is inadvertently removed with the gangue is normally permanently lost from the process and can have a significant economical impact. Even a small decrease in the amount of desired mineral or minerals which are inadvertently floated with the gangue can therefore result in significant economical benefits. Thus, what is particularly needed are depressants useful in reverse flotation systems that depress the flotation of the desired mineral or minerals to a significant degree while having minimal effect of the flotation of related gangue.